Phase steered arrays include a large number of phase-shift elements. The phase and amplitude of each element may be controlled to generate a beam with a particular shape in a particular direction. Typically, the relative amplitudes of each element are fixed, while phase shift settings are adjusted to shape and steer (or point) the beam.
One common phased array implementation uses phase-shift element consisting of a selected number of cascaded binary phase shift components that provide incremental phase shifts. Each phase shift element is set to a selected phase state by a binary control word in which each bit controls a corresponding binary phase shift component, or phase bit, such that the phase response for the element is the sum of the selected phase increments.
To precisely control the beam, the actual phase response of each element must be known precisely. However, phase response is subject to unavoidable errors due to manufacturing discrepancies, and to non-linear materials properties as a function of temperature. Thus, calibration is generally required to provide calibration coefficients for each phase shift element, which can be stored and used during phase steering operations to correct phase response errors.
For some phased array systems the calibration problem is relatively straightforward because the input to each phase shift element may be individually controlled, and its output seperately measure. However, for many systems, space, cost and/or complexity constraints do not allow access to each element, but rather, only the aggregate aperture response (in-phase I and quadrature Q) of all elements in the antenna aperture is available. For these systems, calibrating the phased array can be a relatively involved process, particularly if regular recalibration is required.
Some types of phase shift elements are well behaved in that phase response does not vary significantly over time or as a result of changes in temperature (or other environmental factors). However, the performance of these elements in isolation may differ when they are included in array, requiring calibration to be performed (less conveniently) on an assembled array.
Other types of phase shift elements vary relatively unpredictably over time and/or temperature. For this type of phased array, calibration measurements must be made, and the resultant calibration coefficients estimated, at intervals less than the interval over which the calibration coefficients change significantly.
In either case, current calibration techniques involve empirically estimating calibration coefficients. This approach is disadvantageous in that calibration measurements must be made with special test equipment while the array is off-line. Another significant disadvantage of this empirical approach is that it does not use automated signal processing techniques.
These disadvantages are particularly problematic for arrays in which phase-shifter performance changes with temperature. For such systems, in an effort to extend recalibration intervals, significant design effort is often expended to provide at least some immunity to changes in operational temperatures (for example, by using refrigeration).
Accordingly, a need exists for an improved method of calibrating a phase steered array, which is based on a generalized model of a phased array, and is capable of dynamically updating calibration coefficients while the array is on-line. Preferably, the method would use automated signal processing techniques capable of implementation in equipment generally available in the system of which the array is a component.